''Azospirillum'' is a

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

microaerophilic A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O₂; typically 2–10% O₂) for optimal growth. A more r ...

, non- fermentative and nitrogen-fixing bacterial genus from the family of

Rhodospirillaceae The Rhodospirillaceae are a family of Pseudomonadota. The majority are purple nonsulfur bacteria, producing energy through photosynthesis; originally all purple nonsulfur bacteria were included here.George M. Garrity, Don J. Brenner, Noel R. ...

. ''Azospirillum'' bacteria can promote

plant growth Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant a ...

Characteristics

The genus ''Azospirillum'' belongs in the

alpha-Proteobacteria Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse an ...

class of bacteria. ''Azospirillum'' are

gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

, do not form spores, and have a slightly-twisted oblong-rod shape. ''Azospirillum'' have at least one

flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

and sometimes multiple flagella, which they use to move rapidly. ''Azospirillum'' are

aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cel ...

, but many can also function as

microaerobic A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O₂; typically 2–10% O₂) for optimal growth. A more r ...

diazotroph Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms tha ...

s, meaning, under low oxygen conditions, they can change inert nitrogen from the air into biologically useable forms. At least three species, ''A. melinis'', ''A. thiophilum'', and ''A. humicireducens'' are

facultative anaerobes A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...

, and can live, if necessary, without oxygen. Growth of ''Azospirillum'' is possible between 5 °C and 42 °C and in substrates with a pH of 5 to 9, with optimal growth occurring around 30 °C and 7 pH. Microbiologists use nitrogen-free semi-solid media to isolate ''Azospirillum'' from samples. The most commonly used media is called "NFb".

Discovery and reclassification

The first species described in the genus was originally named ''Spirillum lipoferum'' in 1925 by M.W. Beijerinck. In Brazil, during the 1970s, similar strains of this species were found associated with the roots of grain plants by scientists lead by Dr. Johanna Döbereiner. Her group discovered that these bacteria had the ability to fix nitrogen. Due to this discovery, ''Spirillum lipoferum'' was reclassified in 1978 as ''Azospirillum lipoferum'' by Jeffery Tarrand, Noel Krieg, and Döbereiner, who also added ''Azospirillum'' ''brasilense'' to the genus. By 2020, twenty-one species of ''Azospirillum'' had been described, most of which had been discovered after the year 2000.

Origin of name

The prefix “Azo-” comes from the French word “azote”, which means

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

. This prefix is used to denote the ability of the bacteria to fix atmospheric nitrogen. The ending “-spirillum” refers to the shape of the bacteria which is similar to spiral-shaped bacteria in the genus ''

Spirillum ''Spirillum'' is a genus of Gram-negative bacteria in the family '' Spirillaceae'' of the '' Nitrosomonadales'' of the '' Betaproteobacteria''.Garrity, George M.; Brenner, Don J.; Krieg, Noel R.; Staley, James T. (eds.) (2005). Bergey's Manual ...

.''

Ecological and agricultural significance

''Azospirillum'' are found in freshwater and soil habitats, especially in close relationships with plant roots. Associations with plants are thought to be largely beneficial. Over 113 species of plants in 35 different plant families have been documented to have benefited from association with a species of ''Azospirillum''. In addition to

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

s, the growth of the algae ''

Chlorella vulgaris ''Chlorella vulgaris'' is a species of green microalga in the division Chlorophyta. It is mainly used as a dietary supplement or protein-rich food additive in Japan. Description ''C. vulgaris'' is a green eukaryotic microalga in the genus ...

'' was positively affected by the presence of ''Azospirillum''. Since the 1970s, ''Azospirillum'' strains have been researched for their effects in improving agricultural yields and improving growth of wild plants. In 2009, the first commercial inoculants containing ''Azospirillum'' came on the market, and by 2018, over 3 million doses were applied annually to crops by farmers, mainly in South America.

Plant growth promotion

''Azospirillum'' promote plant growth through a variety of mechanisms. Many ''Azospirillum'' excrete

plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pat ...

s that alter how the roots of plants grow. Affected roots frequently grow more branches and fine root hairs, which may help the plants acquire water and nutrients more efficiently. In addition to these changes, ''Azospirillum'' can also alter the forms of plant nutrients such as

and

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

to make them more available to plants. However, how much nitrogen Azospirillum contribute to crop plants via biological fixation is debated. ''Azospirillum'' also make

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubrica ...

s that protect the plant roots from stresses due to drought and flooding. Plant growth can also be promoted indirectly by ''Azospirillum'' reducing plant disease. ''Azospirillum'' competes with pathogens on the roots for space and for trace nutrients such as

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

. The plants' immune systems can also be primed by ''Azospirillum'' to resist attack by pathogens, a process known as induced systemic resistance.

Known species and genetic diversity

Azospirillum genus harbor over than 20 described species. Despite the remarkable plant growth-promotion properties, less than half of ''Azospirillum'' species have the genome sequenced: ''A. brasilense'', ''A. thiophilum'', ''A. lipoferum'', ''A. oryzae'', ''A. palustre'', ''A. doebereinerae'', ''A. halopraeferens'' and several undescribed ''Azospirillum'' sp. strains. When accessing a phylogenetic tree with all ''Azospirillum'' genomes, it is possible to identify two monophyletic groups, one harboring exclusively ''A. brasilense'' strains and another the remaining species. This strongly suggests a higher differentiation of ''A. brasilense'' from the remaining strains. The second clade also has very high diversity and not enough resolution to determine strains species only using genetic data.